The present invention concerns a timepiece which includes a time base, a generator, formed in particular of permanent magnets rotor and at least one coil, a barrel with a mainspring coupled to said generator in order to drive it, analogue time indicating means also driven by said barrel and operating substantially synchronously relative to the time base, as long as the torque provided by the barrel is greater than or equal to a minimum synchronisation torque, means for braking the generator, and magnetised means having, with the rotor, when the generator is stopped, a static positioning torque allowing this rotor to be kept in a substantially stationary position.
A device of this type is disclosed in European Patent No. 0 822 470. This document relates to an electronic timepiece including a power source formed by a barrel with a mainspring, which is wound manually or automatically, coupled to an AC voltage generator intended to power an electronic circuit of the timepiece, a set of time indicating hands driven by a gear train which is itself driven by the barrel, braking means for adjusting the rotational speed of the generator, for braking or stopping it, and magnets for preventing the generator from rotating, once it has been stopped.
These magnets are placed in proximity to the periphery of the generator rotor. They have a very weak attraction force and exert a weak static positioning torque on the rotor, which, in order not to prevent it from starting again, must not be greater than the torque which the barrel is capable of providing when the power reserve is almost exhausted.
A device of this type has some drawbacks. Indeed, when the minimum synchronisation torque between the hands and the time base can no longer be assured, the watch nonetheless continues to operate. Thus, the mainspring continues to unwind until the power reserve of the barrel is totally exhausted.
Moreover, in the knowledge that the minimum synchronisation torque corresponds for example to winding the mainspring one and a half turns, and that seven complete turns of the winding mechanism winds the mainspring one turn, one has to thus count a minimum of ten turns of the winding mechanism before the watch works in a synchronised manner again.
The object of the invention is thus to overcome the drawbacks of the aforementioned prior art by providing a stopping device for the timepiece which allows a barrel torque to be maintained close to the minimum synchronisation torque when the timepiece is not operating, by stopping the generator as soon as the power reserve is no longer sufficient for the timepiece to operate in a substantially synchronised manner.
The invention thus concerns a stopping device for a timepiece of the aforementioned type, characterised in that the generator braking means are activated to stop the generator as soon as the time indicating means are no longer operating in a substantially synchronised manner with the time base, and in that the magnetised means are arranged such that the static positioning torque, which they exert on the generator rotor is greater than or equal to a reference torque for which the torque exerted by the barrel on the rotor is equal to the minimum synchronisation torque.
In order to obtain a magnet having, with the rotor, a static positioning torque greater than or equal to the reference torque, account has to be taken of the dispersion of quantities such as the stray rotor fields, the rotor manufacturing and assembly tolerances or the magnetisation levels of the magnets. In order to do this, it is necessary to manufacture a positioning magnet with a magnetic flux allowing to generate a static positioning torque with the rotor, which is considerably greater than the minimum synchronisation torque. By arranging such a magnet in a predetermined fixed position in the timepiece, the number of turns of the winding mechanism to be made in order to restart the generator may be considerable.
Indeed, with reference to FIG. 3, the selection of a magnet taking account of the aforementioned uncertainties corresponds to a static positioning torque CT between the magnet and the rotor equivalent to a barrel torque for a winding of approximately 4 turns. According to the invention, the generator is stopped for a winding of the barrel of approximately 1.5 turns.
Thus, when the barrel is being wound, the generator is not started again until the winding-mechanism has been wound a minimum of 17 turns. If it is assumed that the person wearing the watch winds the winding mechanism one and a half turns with each manipulation, he then has to effect a dozen manipulations in order to restart his watch, which may lead him to believe that his watch is no longer working.
In order to overcome this drawback, a second aspect of the invention concerns a stopping device further including means for adjusting the static positioning torque between the magnet and the rotor allowing the torque to be adjusted so that it is substantially equal to the reference torque for which the torque exerted by the barrel on the rotor is equal to the minimum synchronisation torque between the hands and the time base.
A safety factor may be taken in order to take account of the deterioration of the system, in particular, wear of the barrel and problems of gear lubrication or dirt.
This adjustment allows the timepiece to be restarted quickly, after approximately 2 or 3 manipulations, while assuring that the analogue time indicating means are synchronised with the time base.
The timepiece, according to the invention, always operates synchronously, the time indicating means being able to move forward with respect to the time base continuously or by jerks. In a preferred embodiment, the minimum synchronisation torque will be taken to ensure that the time indicating means continuously work in a synchronised manner with the time base.
The invention also relates to the associated method for adjusting the static positioning torque between the magnet and the rotor.